What separates species? Oh boy, do I not have an answer for you! We usually don't have enough fossils to ask that question, and now that we're cataloging so many fossils it's starting to bring up questions like this.

We clearly have two gropus of T.rex skeletons, gracile and robust. Are they different sexes or different species? How about proposed Spinosaurus species, are those just interspecies variation? What about homonin fossils? We can't even agree on the genus, let alone the species.

Well forget fossils, what about living species? Plants have shown an amazing ability to hybridize between very distinct groups, and animals can form ring species where two populations can't interbreed, but can share genes through a string of other populations! How do you classify that‽

Great yt video on the concept of species if you have 15 minutes.